In double heterojunction semiconductor lasers, an active layer in which carrier recombination occurs, resulting in the emission of light, is sandwiched between cladding layers of opposite conductivity type. The cladding layers have larger energy band gaps and smaller refractive indices than the active layer in order to confine light within the active layer. The laser structure includes two opposed, generally parallel facets that are generally perpendicular to the active layer. The facets are coated with a reflective material to produce, with the active layer, an optical resonator in which light resonates to sustain a laser oscillation. The coating on at least one of the facets permits some of the laser light to escape, producing the light output of the laser.
A number of factors limit the power output of a semiconductor laser. Carrier recombination can occur more efficiently at the surfaces adjacent the facets than within the body of the laser. The increased carrier recombination and resulting increased charge carrier density at the facets results in increased light absorption there. That light absorption, in turn, increases the temperature at the facets. If the temperature rise is sufficient, localized melting of the semiconductor materials can occur, resulting in catastrophic optical damage (COD) that destroys the laser.
The power output of a semiconductor laser can be increased without risking COD by providing a window structure as described by Yonezu et al in the Journal of Quantum Electronics, Volume QE-15, August 1979, pages 775-781, the disclosure of which is incorporated herein by reference. In the window structure described by Yonezu, the regions of the semiconductor laser adjacent the facets, i.e., the windows, are heavily doped n-type and the light emitting region, which lies between the windows in the central portion of the laser, is made p-type by overcompensation with a p-type dopant. As a result of this doping profile, the energy band gap in the central portion of the laser is decreased relative to the energy band gap in the windows. The increased energy band gap in the window structures results in reduced absorption of light near the facets, thereby increasing the power level that can be attained without risk of COD.
Although the window structure increases the power output that can be safely produced by a laser, the relatively high doping concentrations associated with the window structure create other problems. For example, when the dopant concentration is relatively high in the light emitting region where carriers recombine and emit light, there is significant light loss due to free carrier absorption, i.e., the absorption of light by charge carriers. In addition, the relatively heavy dopant concentrations throughout the laser structure encourage the flow of leakage currents between the laser electrodes which are generally parallel to the active layer and transverse to the facets. These leakage currents reduce laser efficiency and effectively raise the current threshold at which laser oscillation begins.